1. Field of the Invention
This invention relates generally to electronic amplifier circuits. More particularly, this invention relates to voltage controlled oscillator circuits applied to modulation and amplification functions within radio frequency wireless transmitting equipment.
2. Description of Related Art
As is well known in the art, voltage controlled oscillators are oscillating circuits with a control input signal (current or voltage) that controls the frequency of the output signal of the voltage controlled oscillator. In wireless radio frequency transmission applications the control input signal is used to modulate the phase or frequency of the fundamental frequency of a transmitted signal. Once the frequency is modulated, it is amplified for transmission to a receiver. The amplification is sufficient to overcome signal loss during transmission through a medium, such as the atmosphere, between the transmitting and receiving antennas.
In radio frequency communication, there are three fundamental types of modulation of a fundamental signal for the transmission of intelligence between a transmitter and a receiver. The three types are amplitude modulation where the amplitude of the fundamental signal is changed with the amplitude of the information being transmitted; frequency modulation where the frequency of the fundamental signal is varied with the information; and phase modulation where a phase angle of the fundamental signal is varied with the information being transmitted.
The information may be analog information representing speech, video, or other directly measured data. Alternately, the information may be digital data. In the case of digital data, the digital data representing “1's” and “0's” modulate the signal as amplitude, phase, or frequency shift keying. This further leads to more complex modulation schemes representing symbol codes as different states of the keying signal.
Amplitude and phase modulation such as quadrature amplitude modulation (QAM) and phase shift keying (PSK) are non-constant envelope techniques that provide high spectral efficiency, but require linear amplifiers. Linear amplifiers are known in the art and are classified as A, AB, and B. Class A amplifiers have the highest spectral purity and limit out-of-band emissions.
“Nonlinear Amplifier Effects in Communications Systems,” Liang et al., IEEE Transactions on Microwave Theory and Techniques, VOL. 47, NO. 8, August 1999, pp: 1461-1466, investigates Class-AB amplifiers with a variety of modulation schemes such as non-constant envelope modulation. Liang et al. further describe that when non-constant envelope signals are passed through an amplifier, the amplifier produces both in-band and adjacent channel signals not present in the input waveform, which are a source of interference. An advantage of non-constant envelope signals is their band-limited nature before nonlinear amplification. At modest drive levels in the linear region of the amplifier, there is little spill over of the output signal into the neighboring channels. These tradeoffs under different operating conditions help determine the energy performance of the amplifier modulation combination.
“RF Power Control in GSM Systems for Constant and Non Constant Envelope Modulation Schemes,” Becker et al., Proceedings of the 2003 International Symposium on Circuits and Systems, ISCAS '03, 2003, Volume: 3, pp: III-602-III-605 vol. 3 describes a potential set-up that enables proper control of the output power POUT of an amplifier in presence of magnitude modulation during bursts and supports ramping-up and -down for both GMSK and 8PSK modulation schemes.
U.S. Pat. No. 6,430,213 (Dafesh) describes a generalized quadrature product subcarrier modulation system that applies subcarrier modulation to quadrature modulated signals with constant envelope modulation suitable for efficient sinewave and squarewave subcarrier modulations. The quadrature subcarrier modulation enables the addition of new signals to the in-phase and quadrature phase signals with spectral isolation while maintaining a constant amplitude waveform. The generalized quadrature product subcarrier modulation applies subcarrier modulation to quadrature-multiplexed communication systems without the need to employ non-constant envelope modulation subject to amplitude modulation to amplitude modulation, and amplitude modulation to phase modulation distortions through a nonlinear high power amplifier.
U.S. Pat. No. 6,043,707 (Budnik) illustrates a variable-class linear amplifier that applies a supply type of envelope modulation to an RF power amplifier operating in or near compression at highest envelope amplitudes and transitions gradually to an envelope tracking type of operation at intermediate envelope amplitudes. The amplifier further transitions gradually to a linear class of operation with a constant supply voltage at lowest envelope amplitudes.
U.S. Pat. No. 5,694,433 (Dent) teaches efficient linear power amplification for communications with reduced adjacent channel energy. The amplification involves generating a first signal with the undesired amount of adjacent channel energy and generating a second signal corresponding to the undesired amount of adjacent channel energy. The second signal is subtracted from the first signal to remove the undesired amount of adjacent channel energy.
U.S. Pat. No. 5,426,395 (Cygan) describes power amplifiers that include a feedback element that may be protected from excessive operating power levels by adjusting the feedback element to reduce the drive signals to the power elements. This is accomplished by sensing the output power of the power amplifier to produce a sensing signal. When the sensing signal exceeds a predetermined threshold, a feedback element is adjusted to produce an increased feedback signal. The increased feedback signal is subtracted from an input signal, thus decreasing the drive signal. With the drive signal reduced, the output power is reduced proportionally to the adjustment of the feedback element.
U.S. Pat. No. 6,631,254 (Wilson, et al.) illustrates a radio frequency amplifier using an offset phase locked loop to produce a radio frequency signal from a baseband signal. In order to allow the phase locked loop to process the signal as one of constant amplitude, the signal is amplitude limited prior to application to the phase comparator, and envelope restoration is achieved by envelope-modulating the output in the RF power amplifier stage.